Sorry. How do you know this?
Butler is installed and running in the background.
As far as I remember, Butler is NOT installed and doesn't run in the background.
If you want to use Butler as a command from any path on your PC, then you must add the Butler folder path to your operating system's Path Environment Variable.
If you don't add it to the Path Environment Variable, then your terminal must be in the same folder where you downloaded the Butler executable.
EDIT:
Please review these steps again:
https://itch.io/docs/butler/installing.html
Adding butler to your path
Adding an executable to your path allows you to launch it from anywhere, no matter which directory you're currently in.
On Windows
Follow this article to add the folder where you downloaded butler.exe to your path.
Alternatively, Windows will look into the current working directory when looking for commands
and
First run
To make sure butler is installed properly, open a terminal (cmd.exe on Windows), and type the following command:
butler -V
(that's a capital V, casing matters)
It should print something like that:
head, built on Sep 13 2018 @ 10:59:39, ref 30fe1c38a9611d6b17dc61c7d4fb9582aa369d41